The present invention relates generally to methods and devices for the transportation of ions through one or more pumping stages of a mass spectrometer. More specifically, an apparatus is described which facilitates the separation of neutral gas molecules from ions and passes the ions through one or more pumping stages or regions of a mass spectrometer. Further, the present invention may be used in an apparatus for selecting and/or transporting ions and charged droplets generated from an API source (e.g., Electrospray or Atmosphere Pressure Chemical Ionization, etc.) through a differential pumping region or regions for analysis in a mass spectrometer.
The present invention relates to multipole ion guides for use in mass spectrometry. The apparatus and methods for ionization described herein are enhancements of the techniques that are referred to in the literature relating to mass spectrometry.
Mass spectrometry plays an important role in the analysis of chemical compounds. Specifically, mass spectrometers are useful in determining the molecular weight of sample compounds. Analyzing samples using mass spectrometry consists of three stepsxe2x80x94formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. Several methods exist in the field of mass spectrometry to perform each of these three functions. The certain combination of means used in a particular spectrometer determines that spectrometer""s characteristics.
Mass analysis, for example, can be performed through magnetic (B) or electrostatic (E) analysis. Ions passing through a magnetic or electrostatic field follow a curved path. The path""s curvature in a magnetic field indicates the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. Using magnetic and electrostatic analyzers consecutively determines the momentum-to-charge and energy-to-charge ratios of the ions, and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers. The analyzer, which accepts ions from the ion guide described here, may be any of a variety of these.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the samplexe2x80x94e.g., the molecular weight of sample moleculesxe2x80x94will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile species e.g., insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R.; J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process, MALDI, is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
For example, FIG. 1 depicts a conventional mass spectrometer using an ESI ion source. Ions are produced from sample material in an ionization chamber 104. Sample solution enters the ionization chamber through a spray needle 105, at the end of which the solution is formed into a spray of fine droplets 111. The spray is formed as a result of an electrostatic field applied between the spray needle 105 and a sampling orifice 107. The sampling orifice may be an aperture, capillary, or other similar inlet leading into the vacuum chambers (101, 102 and 103) of the mass spectrometer. Electrosprayed droplets evaporate while in the ionization chamber thereby producing gas phase analyte ions. In addition, heated drying gas may be used to assist the evaporation of the droplets. Some of the analyte ions are carried with the gas from the ionization chamber 104 through the sampling orifice 107 and into the vacuum system (comprising vacuum chambers 101, 102 and 103) of the mass spectrometer. With the assistance of electrostatic lenses and/or prior art RF driven ion guides 109, ions pass through a differential pumping system (which includes vacuum chambers 101, 102 and 103 and lens/skimmer 108) before entering the high vacuum region 1 wherein the mass analyzer (not shown) resides. Once in the mass analyzer, the ions are mass analyzed to produce a mass spectrum.
Many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Victor Laiko and Alma Burlingame to work at atmospheric pressure (Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMS Anal. Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics and mass spectral results are largely independent of the ion production method used.
An elevated pressure ion source always has an ion production region (wherein ions are produced) and an ion transfer region (wherein ions are transferred through differential pumping stages and into the mass analyzer). The ion production region is at an elevated pressurexe2x80x94most often atmospheric pressurexe2x80x94with respect to the analyzer. The ion production region will often include an ionization xe2x80x9cchamberxe2x80x9d (e.g. FIG. 1, ionization chamber 4). In an ESI source, for example, liquid samples are xe2x80x9csprayedxe2x80x9d into the xe2x80x9cchamberxe2x80x9d to form ions.
Once the ions are produced, they must be transported to the vacuum for mass analysis. Generally, mass spectrometers (MS) operate in a vacuum between 10xe2x88x924 and 10xe2x88x9210 torr depending on the type of mass analyzer used. In order for the gas phase ions to enter the mass analyzer, they must be separated from the background gas carrying the ions and transported through the single or multiple vacuum stages.
The use of multipole ion guides has been shown to be an effective means of transporting ions through vacuum. Publications by Olivers et al (Anal. Chem, Vol. 59, p. 1230-1232, 1987), Smith et al (Anal. Chem. Vol. 60, p. 436-441, 1988) and U.S. Pat. No. 4,963,736 (1990) have reported the use of an AC-only quadrupole ion guide to transport ions from an API source to a mass analyzer. A quadrupole ion guide operated in RF only mode, configured to transport ions is described by Douglas et al in U.S. Pat. No. 4,963,736. Multipole ion guides configured as collision cells are operated in RF only mode with a variable DC offset potential applied to all rods. Thomson et al, U.S. Pat. No. 5,847,386 describes a quadrupole configured to create a DC axial field along its axis to move ions axially through a collision cell, inter alia, or to promote dissociation of ions (i.e., by Collision Induced Dissociation (CID)).
Other schemes are available, which utilize both RF and DC potentials in order to facilitate the transmission of ions of a certain range of m/z values. For example, Morris et al., in H. R. Morris et al., High Sensitivity Collisionally-Activated Decomposition Tandem Mass Spectrometry on a Novel Quadrupole/Orthogonalxe2x80x94acceleration Time-of-Flight Mass Spectrometer, Rapid Commun. Mass Spectrom. 10, 889 (1996), uses a series of multipoles in their design, one of which is a quadrupole. The quadrupole can be run in a xe2x80x9cwide bandpassxe2x80x9d mode or a xe2x80x9cnarrow bandpassxe2x80x9d mode. In the wide bandpass mode, an RF-only potential is applied to the quadrupole and ions of a relatively broad range of m/z values are transmitted. In narrow bandpass mode both RF and DC potentials are applied to the quadrupole such that ions of only a narrow range of m/z values are selected for transmission through the quadrupole. In subsequent multipoles the selected ions may be activated towards dissociation. In this way the instrument of Morris et al. is able to perform MS/MS with the first mass analysis and subsequent fragmentation occurring in what would otherwise be simply a set of multipole ion guides.
Ion guides similar to that of Whitehouse et al. U.S. Pat. No. 5,652,427 (1997), entitled Multipole Ion Guide for Mass Spectrometry, use multipole RF ion guides to transfer ions from one pressure region to another in a differentially pumped system. In the source of Whitehouse et al., ions are produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary. Ions are transferred from this first pumping region to a second pumping region through a xe2x80x9cskimmerxe2x80x9d by an electric field between these regions as well as gas flow. A multipole in the second differentially pumped region accepts ions of a selected mass/charge (m/z) ratio and guides them through a restriction and into a third differentially pumped region. This is accomplished by applying AC and DC voltages to the individual poles.
A four vacuum stage ES/MS quadrupole mass spectrometer instrument incorporating a multipole ion guide beginning in one vacuum pumping stage and extending contiguously into an adjacent pumping stage is depicted in FIG. 2. As discussed above, ions are formed from sample solution by an electrospray process when a potential is applied between sprayer 112 and sampling orifice 113. According to this prior art system shown in FIG. 2, a capillary is used to transport ions from the atmospheric pressure where the ions are formed to a first pumping region 114. Lenses 115, 116, and 117 are used to guide the ions from the exit of the capillary 118 to the mass analyzer 119 in the mass analysis region 120xe2x80x94in this case a quadrupole mass analyzer. Between lenses 115 and 117, an RF only hexapole ion guide 121 is used to guide ions through differential pumping stages 122 and 123 to exit 124 and into mass analysis region 120 through orifice 125. The hexapole ion guide 121, according to this prior art design, is intended to provide for the efficient transport of ions from one locationxe2x80x94i.e. the entrance 126 of lens/skimmer 125xe2x80x94to a second locationxe2x80x94i.e. exit 124. Further, through collisions with background (or collisional) gas in the hexapole, ions are cooled to thermal velocities.
In the scheme of Whitehouse et al., an RF only potential is applied to the multipole. As a result, the multipole is not xe2x80x9cselectivexe2x80x9d but rather transmits ions over a broad range of mass-to-charge (m/z) ratios. Such a range as provided by a prior art multipoles is adequate for many applications, however, for some applicationsxe2x80x94particularly with MALDIxe2x80x94the ions produced may be well out of this range. High m/z ions such as are often produced by the MALDI ionization method are often out of the range of transmission of prior art multipoles.
Thus, electric voltages applied to the ion guide are conventionally used to transmit ions from an entrance end to and exit end. Analyte ions produced in the ion production region enter at the entrance end. Through collisions with gas in the ion guide, the kinetic energy of the ions is reduced to thermal energies. Simultaneously, the RF potential on the poles of the ion guide forces ions to the axis of the ion guide. Then, ions migrate through the ion guide toward its exit end.
In the Whitehouse patent, two or more ion guides in consecutive vacuum pumping stages are incorporated to allow different DC and RF values. However, losses in ion transmission efficiency may occur in the region of static voltage lenses between ion guides. A commercially available API/MS instrument manufactured by Hewlett Packard incorporates two skimmers and an ion guide. An interstage port (also called Drag stage) is used to pump the region between skimmers. That is, an additional pumping stage/region is added without the addition of an extra turbo pump, and therefore, better pumping efficiency can be achieved. In this dual skimmer design, there is no ion focusing device between skimmers, therefore ion losses may occur when gases are pumped away. Another commercially available API/MS instrument manufactured by Finnigan applies an electrical static lens between capillary and skimmer to focus the ion beam. Due to narrow mass range of the static lens, the instrument may need to scan the voltage to optimize the ion transmission.
An object of the present invention is to provide an improved multipole ion guide (i.e., quadrupole, hexapole, octapole, etc.) for use in mass spectrometry. More particularly, the present invention provides a multipole ion guide having pre-multipole and multipole guides. Pre-multipole guide is preferably a short (8-20 mm) guide which is used prior to a longer, main multipole and., preferably, between two skimmers (or other optical devices) separating wanted ions from unwanted neutral gas molecules.
In addition, it is an object of the invention to focus the ions toward the center axis of the ion guide while the neutral gas molecules are pumped away through an interstage port of the turbo pump. Efficient differential pumping allows the multipole to be positioned in a region having pressure low enough that ions can be trapped without significant scattering and still high enough to perform collisional cooling. Collisional cooling between ions and background gas can also effect the ion trajectory and ion kinetic energy. The background gas, through cooling the ions, aids in forming an ion beam with reduced energy spread. In some applications it may be desirable to trap the ions in the ion guide for a period of time. If the pressure in this region is too high, ions may be scattered away or fragmented. In a single skimmer system, the effects of this scattering are difficult to manage. In the present invention, though, the dual skimmer pre-multipole is short enough that ions are not trapped in this region. The short period of time spent in this region minimizes scattering and fragmentation. As a result, the ion guide of the invention results in efficient ion transport, increased resolution and sensitivity, and reduced energy spread.
Another object of the pre-multipole is to rapidly transfer ions through a first pressure region into a second, lower pressure region while maintaining a high transmission efficiency. Another object of the pre-multipole is to further cool the ions thereby reducing the ions"" kinetic ions and focusing the ions. Yet another object of the pre-multipole is to provide for the removal of background (or collisional) gas prior to the mass analyzerxe2x80x94such gases may include nitrogen, oxygen, argon, helium, sulfur hexafluoride (SF6), etc. Yet another object of the present invention is to provide a multipole ion guide to facilitate the transmission of ions into a mass spectrometer with minimal scattering and fragmentation of charged particles.
A variety of mass analyzers can be used with the present invention. Such analyzers which accept ions from the ion guide may be any of a variety of single, double, triple, etc., hybrid, hyphenated or non-hyphenated analyzers (e.g., time-of-flight mass analyzer (TOFMS), quadrupole mass spectrometer, quadrupole ion trap, Fourier transform ion cyclotron resonance mass analyzer (FT-ICRMS), ion mobility spectrometer (IMS), Fourier transform mass spectrometer (FTMS).
In one embodiment of the invention, ions and charged droplets generated from ESI or APCI along with neutral gas molecules pass through a capillary into the first pumping region. This region is pumped by a mechanical pump to a pressure of approximately 1-2 mbar. Optionally, the capillary exit and the first skimmer may have an electrical potential difference to push ions forward to the second skimmer while the neutral gas molecules are pumped away.
A second differential pumping region is pumped by the interstage port of a turbo molecular drag pump. The pressure in this region is between 1xc3x9710xe2x88x922 to 1xc3x9710xe2x88x921 mbar. The pre-multipole is preferably located between the first skimmer and the second skimmer in this region, and is preferably operated as RF only. It will separate charged ions from neutral gas molecules when those particles pass through the first skimmer and into the second it skimmer. The electrical field of this pre-multipole redirects ions and forces them to the center of the second skimmer. These ions can then pass through the opening of the second skimmer, while the neutral gas molecules, which are unaffected by the electric field, are pumped away.
Ions passing through the second skimmer enter the main multipole. The pressure in this third differential pumping region in 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x922 mbar. Neutral gas molecules in the third pumping region are pumped away through the main port of a turbo molecular drag pump. Collisional cooling of ions occurs inside the multipole. Cooled ions then enter the mass analyzer chamber for analysis.
A further object of the invention is to provide a multipole ion guide wherein the same potentials (amplitude and frequency) are applied to a pre-multipole guide and a main multipole guide. Alternatively, another object of the invention is to provide a multipole ion guide wherein the pre-multipole has different RF and DC potentials applied thereto than the main multipole (i.e., different amplitudes and/or frequencies) in order to improve ion transmission therethrough, as well as improve the mass selection range thereof.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.